A conductive paste that may be used for substrate hole filling generally comprises a thermosetting resin and a conductive filler added to it, and in such a paste, the conductive filler is kept in contact with each other to ensure the conductivity of the paste.
Recently, a multi-layered substrate has become used for high-density mounting thereon, which is prepared by alternately laminating multiple conductive layers and multiple insulating layers. FIG. 3 is an enlarged, schematic cross-sectional view showing one example of such a multi-layered substrate, in which the reference numeral 31 indicates a conductive layer of copper foil or the like, the reference numeral 31m indicates an inner conductive layer of the type, and the reference numeral 32 indicates an insulating layer of resin or the like.
For inner layer conduction in the multi-layered substrate of the type, a through-hole that runs through the multi-layered substrate is formed in the substrate, a through-hole plate 33 is applied to it, then this is filled with a hole-filling paste 34, and after cured, the excess hole-filling paste 34 is removed by polishing, and thereafter this is covered with a cover plate 35 (for example, see JP-A 4-91489, FIG. 3; Material Technology and Production Process in Built-up Wiring Board (published by the Association of Technology Information, page 60). However, the process is problematic in that the accuracy in pattern formation therein could not be increased since the copper foil on the substrate surface will be thick owing to the twice-plating step of through-hole plating and cover plating therein. Accordingly, it is desired to attain the intended inner layer conduction with a conductive paste alone, without effecting through-hole plating and cover plating. However, the above-mentioned, conventional powder contact-type paste is unsatisfactory in point of the reliability of bonding to the end faces of inner layers, and it could not ensure stable inner layer conduction.
Contrary to this, a multi-layered substrate that may be formed according to the process shown in FIG. 4 is now employed in the art. In FIG. 4, the reference numeral 41 indicates a conductive layer, the reference numeral 42 indicates an insulating layer, and the reference numeral 43 indicates a conductive paste. The multi-layered substrate of the type is produced as follows: A hole is formed in every layer, and filled with a conductive paste 43, and then the layers are laminated in such a controlled manner that the filled holes could be in a line, and thereafter stacked by pressing. However, the multi-layered substrate is also problematic in that it requires a number of production steps and its production costs therefore increase, since the layers are separately holed and the respective holes are also separately filled with paste. In addition, since the production process includes the pressing step, it is still problematic in that the conductivity of the products may fluctuate owing to the fluctuation in the pressing pressure, and the process could not apply to multi-layered substrates with parts buried therein, and therefore its application area is limited.
The present invention has been made in consideration of the above-mentioned matters, and its object is to provide a conductive paste which has good conductivity and good adhesiveness to substrates and which, when applied to a through-hole of a multi-layered substrate, ensures improved reliability of bonding to the end faces of conductive layers in the through-hole. Another object of the invention is to provide a multi-layered substrate using the paste, which enables high-accuracy patterning thereon, not requiring through-hole plating and cover plating as in the related art. Still another object is to provide a method of producing such a multi-layered substrate, in which the production steps are significantly simplified as compared with those in the related art and which therefore enables cost reduction.